1. Field of the Invention:
The present invention relates to an image processing apparatus capable of obtaining a color separation plate whose image quality is superb at a shadow area in particular.
2. Description of the Related Art:
In printing and platemaking fields, there has heretofore been used a method of determining a subtraction signal used for under color removal (hereinafter be abbreviated "UCR") from the minimum value of values indicative of input signals as the three primary colors so as to use the same as a gray component, thereby performing a subtraction of the rate of UCR equivalent for each color.
One example illustrative of such a method will now be described with reference to FIGS. 1 and 2.
On a characteristic chart shown in FIG. 1, there are shown by common axes, a characteristic representing the relationship between an equivalent neutral density Dp and a black-printer value K to be reproduced on a print or printed matter, a characteristic showing the relationship between an equivalent neutral density D.sub.3 created by three forms or plates of Y (yellow), M (magenta) and C (cyan) and a halftone %, and a characteristic showing the relationship between Y, M, C with respect to the black-printer value K and an equivalent neutral density D.sub.4 created by the black printer. Using the characteristic chart referred to above, a point C indicative of the equivalent neutral density obtained by Y, M, C forms subjected to the under color removal and the black printer can be determined from a black-printer value B with respect to a desired density A on the print. In addition, a point Q indicative of the equivalent neutral density obtained by Y, M and C subjected to the under color removal can be determined by setting K equal to 0.
In order to carry out such a process, there is proposed a circuit shown in FIG. 2, for example. In this circuit, Yj, Mj and Cj, each indicative of the equivalent neutral density, of Y, M and C as the three primary colors are inputted to a minimum value detection circuit 42 and supplied as address data to look-up tables 46Y, 46M and 46C. The minimum value detection circuit 42 serves to determine a signal indicative of the minimum value of the equivalent neutral density, which is selected from Yj, Mj and Cj each indicative of the equivalent neutral density, i.e., the minimum value of Yj, Mj and Cj each indicative of the equivalent neutral density. The minimum value thus determined is applied to a black-printer generating circuit 44 used to generate a black-printer value. The output KS of the black-printer generating circuit 44 is supplied to each of the look-up tables 46Y, 46M and 46C as address data. Each of the look-up tables 46Y, 46M and 46C is used to store therein data about the rate of the UCR, which has previously been determined by an experiment. In addition, the look-up tables 46Y, 46M and 46C respectively output UR.sub.Y, UR.sub.M and UR.sub.C each indicative of the rate of the UCR in accordance with the input KS as the address data and Yj, KS and Mj, and KS and Cj.
The above-described method has, however, the problem that noise independently produced for each color at each of the input signals exerts an influence on other color forms or plates, thereby greatly deteriorating the quality of an image of each color separation form or plate at a shadow area in particular.